The present invention relates to shaping heat-softened sheets, and is particularly useful in improving the method and apparatus for shaping a series of glass sheets on a mass production basis by the roll forming method. Hence, while the description of an illustrative embodiment will describe the forming of shaped glass sheets, it is understood that this invention also applies to forming sheets of any heat-softenable material.
According to the roll forming method of shaping glass sheets, a series of glass sheets moves along a given conveyor path through a heating furnace to arrive at a position between upper and lower sets of rotating forming rolls at a glass shaping station. One set of forming rolls has a curvature that is complementary to the curvature of the other set of forming rolls.
When each glass sheet reaches a position between the sets of rotating forming rolls, the lower set of rotating forming rolls moves upward relative to the other set from a retracted position below said conveyor path to lift the glass sheet from the conveyor path into a sheet engaging position relative to the upper set of rotating forming rolls. The sheet moves between the rolls while engaged at both its upper and lower surfaces for a sufficient time to impress the complementary shapes of the roll sets onto the glass sheet. The sets of forming rolls are then separated and the shaped glass sheet is lowered while supported on the lower set of rotating forming rolls until it is redeposited on the conveyor path.
The rate of cooling determines whether the glass sheet is annealed, partly tempered or fully tempered.
When the next heat-softened glass sheet in the series arrives between the sets of upper and lower forming rotating rolls of the shaping station, the lower forming rolls are again moved upward toward the upper forming rolls to sheet engaging position.
The roll forming method has made it possible to shape a succession of sheets without interrupting the continuous movement of the sheets from the furnace to the cooling station. Such continuous sheet movement has resulted in increasing the rate of production of shaped glass sheets and also facilitated the shaping and tempering of thin glass sheets.
Glass sheets have been shaped and tempered while suspended from tongs. The latter tended to weaken and distort the glass at the points of tong gripping. In an effort to improve both the strength and optical propeties of glass sheets, other techniques were developed to avoid the problems of tongs.
Prior to the development of the roll forming method, horizontally disposed glass sheets had been bent to shape on a mass production method by moving a series of glass sheets through a furnace and stopping the movement of each sheet in turn when the latter was aligned between a pair of shaping members. Stopping the glass sheet movement at the shaping station necessarily increased the time it took for each glass sheet to transfer from the furnace to the cooling station. Therefore, it was necessary to heat the glass before it left the furnace to temperatures sufficiently high to insure its arrival at the cooling station at a temperature sufficiently high to insure its being tempered. Such high temperature heating softened the glass surfaces sufficiently to cause the glass to stick to one of the shaping molds and to develop surface irregularities that impaired the optical properties of the shaped glass.
According to the prior art, cold air was blasted against the major surface of the glass before the latter left the shaping station, either to harden the glass sheet surface or to help separate the glass sheet from one of the shaping molds. U.S. Pat. No. 3,265,484 to Ritter, U.S. Pat. No. 3,361,552 to Ritter and U.S. Pat. No. 3,488,178 to Welker and Ritter disclose the application of cool air against one or both major surfaces of a glass sheet at a press bending station where the glass is completely stopped before the glass moves into the cooling station where a partial or complete temper is imparted. The portions of the glass resting on the glass support structure at the shaping station are obturated from the cool air so that there is non-uniform surface cooling. This can result in undesired warpage or iridescence in polarized light.
The need for curved glass sheets free of surface marking resulting from overheating the glass prior to shaping to insure that the glass sheet arrives at the cooling station at a sufficiently high temperaure to be tempered, yet without excessive heating that causes surface marking has been recognized and new methods and apparatus that do not necessarily require glass sheets to be stopped at a shaping station have been developed. Examples of such development are found in U.S. Pat. No. 3,545,951 to Nedelec and U.S. Pat. No. 3,701,644 and 3,856,499 and 3,871,855 to Robert G. Frank. The latter three patents illustrate apparatus for shaping continuously moving glass sheets by the so-called roll forming method.
The roll forming method has improved the efficiency of the fabrication of shaped glass sheets for use in automobiles, particularly in that it has avoided marks that resulted from the need to overheat glass sheets before press bending and tempering. The apparatus covered by the various Frank patents is not only capable of producing glass sheets to various complicated curved configurations at a much more rapid rate than the prior art, but also has made it possible to reduce the proportion of shaped, tempered glass sheets that are rejected for optical defects due to initial overheating. Since each glass sheet moves from the furnace exit to the cooling station in less time when it is shaped by roll forming than when the sheet is stopped completely for shaping, less initial heat is needed and to compensate for the cooling that takes place in the glass between the furnace and the cooling station.
The roll forming apparatus of the type depicted in the Frank patents is especially adapted to produce a large number of identical parts on a mass production basis without requiring any loss of time to shut down equipment during a run. It has been noted, however, that as higher production rates were attained, that various defects began to develop in the shaped glass sheets that were produced, even when the glass sheets were not overheated initially as evidenced by good optical properties of and conformance to desired shape of glass sheets early in the production runs of given patterns. The present invention provides means for reducing the occurrence of these defects as high speed production runs continue.
In roll forming non-rectangular glass sheets, particularly those having an extremity of reduced width along one longitudinal side edge of the glass that forms an acute angle that merges into either a point or a truncated extremity, it is extremely difficult to control the shape of the glass in the pointed or truncated extremity over a sustained period of time even though the glass sheets develop a shape that conforms with specifications during the early stages of a production run. This problem becomes magnified when one attempts to use roll forming apparatus at higher production rates than those for which the apparatus was originally designed.